1. Field of the Invention
The present invention relates to a method for producing magnetic particles and more particularly to a method for producing magnetic particles having structures that the peripheries of fine particles are surrounded by insulating materials.
2. Description of the Related Art
In recent years, compact and light-weight communication devices have been more increasingly developed as represented by portable telephones. Accordingly, parts mounted on the communication devices have been requested to be more compact and lighter. With the development of such small-sized communication devices, operating frequencies tend to rise.
For coping with the situation that the operating frequencies of the communication devices are liable to high, has been carried out an attempt the electric resistance of magnetic materials used for individual components of the communication devices such as transformers, inductors or magnetic heads is raised to reduce eddy current flowing through the magnetic materials themselves. As such magnetic materials, an amorphous alloy film in which metal and ceramics are sputtered at the same time to disperse the ceramics has been proposed in Japanese Patent Laid-Open Publication No. sho 60-152651 and Japanese Patent Laid-Open Publication No. hei 4-142710.
In a recent communication environment where the communication devices are used mutually at close positions, the high frequencies of the communication devices may possibly cause a communication quality to be deteriorated. Therefore, the components of the communication devices employ magnetic materials having higher magnetic permeability in a high frequency area to absorb unnecessary radio waves generated from the communication devices so that the deterioration of communication quality is prevented.
In order to realize the high magnetic permeability by such a radio wave absorber used in the communication device, the magnetic material forming the radio wave absorber needs to have a high saturation magnetization as well as a high electric resistance and the anisotropic magnetic field and the magnetostriction of a magnetic member need to be low. In recent years, people have paid their attention to a nanogranular structure as a structure of a magnetic material to achieve these properties at the same time. The magnetic material has a structure that the surface of each magnetic particle constituting the magnetic material is surrounded by a thin insulating film and these magnetic materials are connected together in a network form. With such a structure, grain boundary layers having a high resistance are formed between the magnetic particles to generate a high electric resistance and a high magnetic permeability is realized in a high frequency area while the magnetic particles, which are not isolated particles such as those of superparamagnetism, are brought to close to one another.
In recent years, as for the magnetic thin film having the nanogranular structure, Japanese Patent Laid-Open Publication No. hei 10-241938 discloses that a nanogranular thin film composed of a cobalt (Co) group has a magnetic permeability not higher than several hundred MHz. Further, according to a report concerning a magnetic permeability (J. Appl. Phys., Vol. 87, No. 2, 15 (2000), P187), a Co alloy thin film has a similar magnetic permeability.
However, the magnetic materials having the nanogranular structures which have been heretofore reported have been inconveniently limited to thin films using a sputtering method.
Reported values on the magnetic thin films having the nanogranular structures have been directed only for study of the materiality of one particle but for bulk materials. Further, an application study utilizing the magnetic thin film has been rarely performed. It has been especially difficult for the thin film to make a property of a radio wave absorber for electromagnetic waves in neighboring places compatible with a property as a radio wave absorber for electromagnetic waves in remote places.
Further, when the magnetic material having the nanogranular structure is manufactured, it is estimated that a thick film not thicker than about 100 μm can be manufactured by repeatedly carrying out sputtering operations using a method for manufacturing the magnetic thin film having the nanogranular structure. However, it takes high cost and long time to produce the magnetic material, and accordingly, this method for manufacturing the magnetic material is not realistic from an industrial point of view.